Secrets of the Viking Sun-Compass -- After Dark

Viking sailors may have been able to navigate after sunset, despite having no magnetic compasses and relying on sun compasses, says a team of researchers from Hungary and Sweden.

The trick may have been to combine the power of a sun-compass, which is a sort of modified sundial, with that of sunstones to create what they are calling a "twilight board."

"The recently identified calcite crystal found between the navigational tools of 16th-century European ship wreck hints that they could play some role in marine navigation" reports Balázs Bernáth and his colleagues in the Proceedings of the Royal Society A.

To test out if such twilight boards might have existed, the team examined a fragment of an 11th-century dial found in Uunartoq, Greenland, and attempted to extrapolate its features into something that would allow Viking navigators to detect the position of the sun from the twilight glow on the horizon passing through two sunstones.

If the difficult technique was employed, it could have allowed navigation well after sunset, since the twilight glow can last all night long at high latitudes in summer. Sunstones are recorded in Viking sagas as being employed to navigate on cloudy days, although what the stones were and how they worked remains lost knowledge.

The first step in regaining that knowledge was taken a few years ago by physicist Guy Ropars of the University of Rennes in France. Ropars and his team experimented with a potential sunstone that was recovered from a British ship which sunk in 1592. They applied partially polarized laser light on the calcite crystal -- similar to the kind of light that would make it through clouds (direct, unfiltered sunlight is not polarized).

By turning the crystal they found that a single angle where the crystal splits the light and transmits both unpolarized and polarized light beams at equal strength. This angle of equal brightness of the two beams of light in the crystal could be used by navigators to then determine the angle of the sun in the sky -- without actually being able to see the sun.

The twilight board would have used the same approach, argues Bernáth, but instead of being used during the day to navigate under clouds, it would use the light in the sky after sunset, along with a specially marked dial, perhaps one like that found in Uunartoq.

"Developing a twilight board is surely not beyond the capabilities of sea-faring people," Bernáth opined. "Sunstones are mentioned in written sources and they could be used during civil twilight, although it is not trivial how one can accurately estimate the position of the sun with them."

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